The problem of fixing a sheet member to a surface of a support body in a sealed but demountable manner arises very often in practice. In general, such a problem could be solved by cementing the sheet member on to the surface of said support body. However, such a system is not generally advisable if the sheet member has to be easily detachable, and if this later is subjected, as for example in the case of a diaphragm, to fatigue stresses which tend to detach it from the support body.
In the aforesaid cases it is preferred to clamp a peripheral portion of the sheet member between the support body and a fixing element removably connectable to the support body. Said peripheral portion can be constituted by an outer flange lockable on to the support body by means of a backing flange, or by a bead arranged to engage a corresponding groove provided on the support body and locked therein (see for example U.S. Pat. No. 3,152,031) by inserting an insert lockable on the support body.
Although particularly advantageous when wishing to connect a sheet member to a support in a sealed but demountable manner, both the fixing systems described in the preceding paragraph are not free from drawbacks in that in all cases they require the use of the aforesaid fixing element, the assembly and removal of which are not always simple, and of which the sizes are sometimes such as to lead to an increase in the dimensions of all the possible surrounding members.
In order to obviate the aforesaid drawbacks, it is known to connect a sheet member to a support body in a sealed manner by means of a bead of trapezoidal shape (see for example U.S. Pat. No. 4,024,770) rigid with the sheet member and housed in a groove of the same shape provided on the support body.
Such a fixing system does not require the use of any additional fixing member, and is particularly effective in the case in which the sheet member, or at least that part of it in proximity to said bead, is subjected to forces which urge it against the support body or tend to cause it to slide therealong, but proves totally ineffective if forces are applied to the sheet member which tend to detach it from the support body.
This is due to the fact that in order to be able to be inserted into the relative trapezoidal groove, said bead must necessarily be constructed by material which deforms elastically with relative ease. Consequently, it is not able to react to even low intensity forces which tend to extract it from the relative groove.